Twice in the last week my gas fired baseboard hot water boiler's pressure relief valve has discharged during the night with steam and hot water, mostly steam.

This has happened before, last season I replaced the house water supply pressure regulator valve and the problem seem to go away at that time.

When the pressure relief valve blows and I get down there to look at it the burner is off and there are no zones open. Also, the circulator pump is not running. I have three zones on a 100k btu boiler with a zone valve control box. What are the possible causes?